This invention relates to electrical generators. More particularly, the present invention relates to an improved energy density homopolar generator in which inertial energy is stored over a long period of time for delivery to a load in a short period of time.
Homopolar generators (HPGs) are well known and described in the literature. For example, the article entitled "The Design, Fabrication, and Testing of a Five Megajoule Homopolar Motor-Generator", authored by W. F. Weldon, et al., and presented at the International Conference on Energy Storage, Compression and Switching in Torino, Italy in November of 1974, presents an overview of HPGs in general. The article "Compact Homopolar Generator", authored by J. H. Gully, which appeared in the IEEE Transactions on Magnetics, Volume MAG-18, No. 1, January, 1982, discloses an improvement in the design of a HPG.
These and other recent design changes in homopolar generators have resulted in a substantial improvement in the HPG energy density which could be achieved over typical prior art existing designs. HPG's are now being considered as power supplies for electromagnetic launchers and other applications in the field rather than in the laboratory. Consequently, energy density has now become important. One such improved design is known as the All Iron Rotating (A-I-R) homopolar generator, and represents a design approach which eliminates, as far as possible, the non-rotating parts of the generator, concentrating the mass of the generator in the energy storing parts (rotor(s)). The rotor of the A-I-R HPG weighs typically about 1,600 pounds out of a total generator weight of 3,400 pounds, or about 47% of the total mass. Such a machine stores about 4 kilojoules (KJ) of energy per kilogram (kg) of machine mass. This may be compared to the rotor of a typical prior art homopolar generator which occupies approximately 10% of the generator mass. Such a typical prior art HPG has a total energy capacity of approximately 5 MJ (megajoules) for a specific energy density of less than one Kj/kg than one (kilojoules per kilogram).
Since the A-I-R homopolar generator approach now uses almost half of its weight to store energy it will never be able to improve its specific energy density by more than a factor of approximately two by following the design approach of minimizing the stator mass. Once the generator mass is reduced near the rotor mass, the only apparent method for increasing energy density is to increase the rotational speed of the rotor. As in any generator, the rotational speed of the rotor is controlled by an external input drive force, such as that provided by a connected hydraulic or electric motor.
While it may be possible to theoretically increase the specific energy density of the generator by increasing the rotational speed of the motor, such a design approach presents design limitations caused by the wear rate of the electrical brushes used as current collectors on the rotor surface. These electrical brushes are typically constructed of a sintered copper-graphite composite, and are poor thermal conductors making it difficult to remove the frictional heat created at the brush-to-slip ring interface.
This surface heating of the electrical brushes is a result of two phenomena, coulomb frictional heating and heating due to the brush-to-slip ring interface voltage drop. As the surface temperature of the brush reaches the melting temperature of the binders in the copper-graphite sinter, substantial wear occurs. Because of the physical phenomena occurring at the interface between the brushes and the slip ring surface, it has been found for a typical HPG application that rotor surface velocities cannot exceed approximately 220 meters per second. Although the technology is attemping to discover exotic brush materials which are more amenable to the removal of this surface heating, it is anticipated that brush wear will continue to limit HPG rotor speeds.
Since it is desirable to continue to increase the specific energy density of homopolar generators, and since brush wear prevents increasing rotor speed (the most promising route to increase energy density), it would be attractive to decouple the brush/slip ring speed from the allowable rotor tip speed.
Another way to improve the effective overall energy density of an HPG powered system is to reduce the size of auxiliary components or combine them with other components. The concept of a self excited HPG achieves this by using the generator output current to provide field excitation for the HPG itself. Energy stored inertially in the rotor is converted to energy stored magnetically in the field coil. In some cases the ferromagnetic rotor and stator material can be eliminated allowing the magnetic field to rise faster and to higher levels than can be achieved in ferromagnetic machines. This in turn allows the machine to develop higher output voltage and therefore higher power density as well. Of course, the elimination of ferromagnetic material can also reduce the weight of the machine, also improving the energy density.
The arrangement described above is particularly attractive if the HPG is to be used to charge an intermediate inductive energy store such as is such to power an electromagnetic railgun. In this case the HPG field coil can serve as the inductive store so that the total energy stored inertially in the rotor is transferred to energy stored inductively in the field coil and then transferred to the load. Not only does this eliminate the need for a separate excitation supply, but it eliminates a separate intermediate storage inductor as well. Such a machine is described in U.S. Pat. No. 4,503,349 by Miller.
A problem with self excited, air core HPG's such as that described in the Miller patent is the balance of energy required in the field coil for excitation versus the energy originally stored in the rotor. In order to take advantage of the higher magnetic flux density (and therefore higher HPG power density) that can be achieved in the air core (nonferromagnetic) excitation coil, the energy required typically exceeds the energy that can be stored in rotor(s) fitting inside the coil and operating at conventional (about 200 m/s) surface speeds dictated by present day brush technology. This usually forces the rotors to be operated at higher than standard surface speeds in order to increase rotor energy density to the point needed to excite the field coil(s). This in turn compromises the durability of the HPG by causing the brushes to operate well above their capabilities.